Optical parametric amplification is a non-linear optical process where light at one wavelength, the pump wavelength, is used to generate light at two other (longer) wavelengths in a non-linear (NL) optical material with a non-vanishing second order non-linear susceptibility. Optical gain is established at the two generated wavelengths, conventionally referred to as the signal wavelength and the idler wavelength. The sum of the energies of a signal photon and an idler photon are equal to the energy of a pump photon. There is no fundamental physical distinction between the idler photon and the signal photon. However, it is customary to refer to the shorter of the two generated wavelengths as the signal and the longer generated wavelength as the idler. An optical parametric oscillator (OPO) comprises an optical cavity containing a NL optical material which provides optical amplification when pumped by a beam of laser radiation at a pump frequency from a pump source.
An optical parametric amplifier (OPA) is a laser light source that emits light of variable wavelengths by an optical parametric amplification process that can also be referred to as Optical Parametric Generation (OPG). OPAs are typically used to amplify a pulse that is compatible with the particular OPA. A double-pass OPA is formed by including an optical delay generator in the OPA system that can almost double the total gain extracted from a single pass OPA by transferring pump pulse energy to both a first and a second signal or idler pulse, instead of energy transfer to only a single signal/idler pulse.
A double-pass OPA is simply a single pass OPA with a mirror at the output that sends both of the respective pulses back through the NL crystal a second time. A double-pass requires the pump and signal or idler pulse to be re-synchronized with each other since passing the pulses through the NL crystal dephases them with respect to each other. The OPA gain is instantaneous. The gain coefficient is proportional to the pump pulse intensity, resulting in the gain for the OPA changing along the pump pulse profile.
Accordingly, for a double-pass OPA system temporal alignment of the signal pulses and pump pulse is needed for maximum gain. The first and second pass pulses to be amplified (signal or idler) both need to temporally overlap with the pump pulse while these pulses are inside the NL optical material. Temporal synchronization gets more difficult as the respective pulse widths get shorter.
Optimum pulse spacing between signal/idler pulses is required for best possible system performance (i.e., maximum output power). FIG. 1 is a depiction that demonstrates an ideal time synchronization that provides optimum pulse spacing between first and second pass signal/idler pulses and a pump pulse for a double-pass OPA to provide maximum energy transfer needed for maximum power output to the signal/idler pulses. Specifically, ideally the first and second pass of the signal/idler pulses are placed as close as possible to the center of the pump pulse without overlapping one another. As described below, the first and second passes of the signal/idler pulses cannot be directly overlapped with one another within the pump pulse because at high gain they would be operating in the depleted gain region.
The larger Gaussian envelope in FIG. 1 represents the time profile of the pump pulse shown as an example 7 ps wide pulse. The two smaller Gaussian profiles (shown each as 1 ps wide pulses) represent the temporal profile of the signal/idler pulses. One of the smaller profiles represents the first pass through the OPA while the other smaller profile represents the second pass through the OPA. The respective signal/idler pulses are delayed against each other relative to the pump pulse (shown as a 4 ps relative delay) by an optical delay so they do not both pass through the pump pulse in the same temporal position, since the second signal/idler pulse must be shifted to a fresh portion of the pump pulse where the energy of the pulse has not been depleted, so the maximum possible energy transfer can occur.
Conventional double-pass OPAs utilize multi-component optical-mechanical system-based optical delay generators for the required time synchronization which comprise beam splitters, mirrors, and translation stages. Such optical-mechanical system-based optical delay generators are bulky, and are generally difficult to obtain good results from. Involved measurements are also needed to ensure that the delay is accurate to the level required. Moreover, conventional multi-component optical delay generators can be difficult to work with, and are susceptible to being knocked out of alignment.